WASHINGTON (AP) _ The Tower commission investigating the Iran-Contra affair may have made a mistake in saying Lt. Col. Oliver North was the author of a mysterious diagram, a commission spokesman said Monday.

''We don't know that it's a mistake. We don't know who did it in retrospect,'' said commission spokesman Herb Hetu. The report says North sketched the diagram.

The diagram, which appears to suggest a flow of money and weapons in a private network run by North to aid the Contra rebels in Nicaragua, is included in an appendix to the report that was released last week.

It was sketched onto the bottom of a typed note from North's secretary to Phil Mabry, a Texas businessman who has been active in seeking support for the Contras.

Sources on Capitol Hill said the diagram widely is perceived to have been drawn by Mabry himself, who carried the note around with him last year when he was seeking congressional support for the Contras.

Mabry did not return phone calls on the matter.

The sketch shows arrows linking North's name to those of businessman Bob Owen, under whose name is written the word ''weapons,'' and conservative activist Andy Messing, under whose name is the word ''funds.''

Other arrows link those names to that of Linda Guell, an official of the conservative foundation Western Goals, and of Causa, an international foundation funded by members of the Rev. Sun Myung Moon's Unification Church.

Messing, whose National Defense Council Foundation has been active in assisting refugees in Central America, said Monday that he may sue the Tower commission for falsely implying he was involved in financial or weapons transactions, thus jeopardizing his reputation and his ''physical safety'' when he travels in Central America.

''I've told hundreds of reporters that I do nothing but food and medicine'' assistance, he said. ''I never took any funds from North.''

He said his group provided 110 tons of food and medicine in Central America, seven tons of it to the Contras fighting to overthrow the Nicaraguan government, but never any weapons.

He said he knows Owen and Guell but has no dealings with them.

Messing said a reporter who contactd Mabry told him Mabry drew the diagram, and that North's wife and North's secretary, Fawn Hall, have also told him that it was not North's sketch.

Congressional sources said the handwriting looked like Mabry's and the diagram coincided with a scenario that he outlined to members of Congress last year.

Hetu said a commission staff member assumed the diagram was sketched by North, but that that conclusion is now in doubt.

However, Hetu said, no efforts are being made to check the veracity or amend the report because ''it didn't materially affect our report.''

The Tower report, which focused mostly on the U.S. arms sales to Iran, did not elaborate on what the sketch might mean.

Causa executive vice president Thomas Ward said his organization had no idea what the diagram meant and had never had any contact with North, Messing, or Owen, although Guell did attend a Causa conference in 1985.

He said his group is involved in educational projects promoting democratic principles and has provided some nonmilitary supplies to Central American refugees.

Guell, who left Western Goals and now works part time for Sen. Strom Thurmond, R-S.C., has not returned a reporter's calls in recent weeks. Western Goals, founded by the late Rep. Larry MacDonald, D-Ga., now is run by conservative fund-raiser Carl ''Spitz'' Channell who helped rally funds and public opinion for the Contras.

Owen, who cited his Fifth Amendment rights against self-incrimination in refusing to testify before congressional committees on the Iran-Contra matter, could not be reached for comment on the Tower report.